Historically, because it is expensive to install single pair telephone lines in sparsely populated rural areas, from two to eight (and sometimes more) rural telephone subscribers have had to share the same single pair telephone line. Although the number of rural subscribers who have had to share such "multiparty" telephone lines has decreased in recent years, it has been estimated that about 5% of North American telephone subscribers still have only multiparty service.
Various analogue circuitry systems have been devised in an effort to enable from two to eight subscribers to communicate simultaneously over the same single pair telephone line without interfering with one another's communication. However, such systems are typically expensive to install and expensive to maintain. Experience has shown that the equipment must be continually adjusted to maintain acceptable performance levels, due to the drifting of carrier frequencies and the various filters which separate the high and low voice bands which make up each telephone communication channel. The lack of suitable diagnostics to assist in identifying and correcting alarm or breakdown conditions has also been problematic. Essentially, a service disruption caused by a faulty repeater located within such a system has required the system maintenance personnel to visit every signal repeater located between the telephone central office and the subscriber(s) experiencing the fault until the faulty repeater is located and repaired. This is obviously a time consuming, frustrating and costly endeavour. Moreover, telephone companies are currently attempting to upgrade their networks by phasing out analogue equipment in favour of more reliable digital equipment. Accordingly, telephone companies are not enthused about the prospect of investing in the installation of more analogue equipment.
Multiplexed digital subscriber systems have recently been developed which enable four wire communication (i.e. two wires for communication in the transmit direction and two wires for communication in the receive direction). However, such systems conventionally provide for communication with groups of six subscribers. Accordingly, such systems are not well suited to rural use, because rural telephone subscribers are not normally found in clusters of six. Moreover, because such systems would require the installation of additional cable pairs over and above the existing single cable pairs which currently service multiparty rural telephone subscribers, installation costs would increase.
Digital concentrators have also been considered to provide service to multiparty subscribers. However, digital concentrators are relatively sophisticated, expensive products best suited to situations in which a large number of subscribers are found in a concentrated area. The digital concentrator essentially serves as a local switch which reduces the number of lines required between the telephone central office and the rural telephone office at which the digital concentrator is situated. The cost factor, coupled with the fact that rural telephone subscribers are not normally found in large concentrations, renders digital concentrators unsuitable as a widespread solution to the problem of enabling simultaneous multiparty communication over existing rural single pair telephone lines.
Rural telephone subscribers are placing increasing pressure on telephone companies to provide them with telephone service equivalent to that enjoyed by urban telephone subscribers. That is, rural telephone subscribers wish to be freed of the inconveniences of multiparty service. This is particularly true of the increasing number of sophisticated rural telephone subscribers who wish to use the telephone lines for data communication purposes to assist their business endeavours. Reliable data communication over shared multiparty lines is not possible. One way of upgrading service to such subscribers would be to run dedicated single pair telephone lines to each rural subscriber as is done in urban areas. However, the cost of running a cable over an extended distance to service a relatively few subscribers cannot be justified. Moreover, rural subscribers have objected to the disruption caused by cable plowing programs. The telephone companies thus desire a solution which will allow them to upgrade service to rural customers without the installation of additional telephone lines.
The present invention is a signal repeater which may be utilized by a digital subscriber carrier system capable of allowing up to four telephone subscribers to communicate simultaneously over an existing single pair telephone line. The system thus allows telephone companies to replace dissatisfactory rural multiparty service with the desired individual line service without necessitating the installation of additional cable pairs. The system consists of a central office terminal, a subscriber terminal and the signal repeater which is the subject of the present application.